This invention relates to video-on-demand systems which send multiple streams of video data to respective viewing devices. In the prior art, one such system is described in U.S. Pat. No. 5,583,561 which is entitled “Multi-Cast Digital Video Server Using Synchronization Groups”.
An overview of the above prior art system is shown in FIG. 1 of patent 561. There, several videos are stored in a remote video library 10 which is coupled via a communications channel to a video server 12; and, the video server 12 is coupled through a network interface circuit 18 to a plurality of viewing devices 22, 24, and 26 on a network 20. To receive a particular video at a particular viewing device, a request is sent by a client via a telephone to the video server.
In response to the client's request, the video server 12 retrieves the requested video from the video library 10. Thereafter, the retrieved video is sent as a stream of video data from the video server 12 thru the network interface circuit 18 to the particular viewing device on the network 20 which is for the client who requested to see the video.
One method for passing the stream of video data from the video server 12 thru the network interface circuit 18 is disclosed in detail in the above-referenced patent 561. Also, an alternative method is disclosed in a pending patent application which has Ser. No. 09/318,987 and is entitled “Scaleable Video System Having Shared Control Circuits For Sending Multiple video Streams To Respective Sets Of Viewers”. All of the details in the above patent and patent application are herein incorporated by reference.
In the above patent and patent application, the video which is retrieved from the video library can be stored within the video server 12 in a semiconductor memory. Thereafter, each time the video is sent as a stream of video data to a particular viewing device, the source of the stream can be the video in the semiconductor memory. Such a video data stream is herein called a memory-based video stream.
Alternatively, the video which is retrieved from the video library can be stored within the video server 12 on a magnetic disc. Thereafter, each time the video is sent as a stream of video data to a particular viewing device, the source of the stream can be the video on the disc. Such a video stream is herein called a disc-based video stream.
The above memory-based video streams and disc-based video streams are disclosed by a U.S. patent application which is entitled “Video On Demand Video Server Disk/Memory Streaming Selection Methodology”, filed on Nov. 16 of 2000, having Ser. No. 09/714,072. In that patent application, the path thru the video server which is followed by each disc-based video stream is described in conjunction with FIG. 2; and the path thru the video server which is followed by each memory-based video stream is described in conjunction with FIG. 3.
However, in the above patent application, no disclosure is made of how to seamlessly replace all disc-based video streams of a particular video with corresponding memory-based video streams. By such a seamless replacement is herein meant that while the video streams are being sent to respective viewing devices, the source of the video is changed from the disc to the semiconductor memory with no interruption, or discontinuity, in the video streams that are sent to the viewing devices.
Likewise, in the above patent application, no disclosure is made of how to seamlessly replace all memory-based video streams of a particular video with corresponding disc-based video streams. By such a seamless replacement is herein meant that while the video streams are being sent to respective viewing devices, the source of the video is changed from the semiconductor memory to the disc with no interruption, or discontinuity, in the video streams that are sent to the viewing devices.
One reason for wanting to replace all disc-based video streams of a particular video with corresponding memory-based video streams can be understood by considering what happens within the video server as the number of disc-based video streams gets larger and larger. As that occurs, the number of times which the video server must read the video from the disc increases; and consequently, the total disc-read workload on the video server increases. Eventually, the disc-read workload can get so large that the video server cannot keep up with all of the disc-based video streams that need to be sent. To reduce the disc-read workload on the video server, all disc-based video streams of a video which has a large number of viewers can be replaced with memory-based video streams.
However, a problem with memory-based video streams is that a large amount of semiconductor memory is needed to store just one complete video, and semiconductor memory is much more expensive than a disc. For example, a typical two hour video takes about three-billion bytes of semiconductor memory storage. Thus, if one or more videos are each being streamed to only a small number viewing devices, then to send those streams as memory-based video streams is not economical.
Accordingly, a primary object of the present invention is to provide a method of seamlessly replacing disc-based video streams with memory-based video streams in a video-on-demand system.